A method and apparatus is provided for increasing sensitivity for near-real-time detection of very low concentrations of diffusely distributed trace vapors in a carrier medium. Before admitting the trace vapor bearing carrier medium into a near-real-time GC/MS trace vapor detection system, the concentration of the trace vapor in the carrier medium, air in this instance, is increased by passing it through a membrane gas separator. The gas separator preferentially passes a portion of the trace vapor and rejects all but a very small portion of the carrier medium. The sample, highly concentrated in trace vapor with respect to the carrier medium after passing through the gas separator, is then compressed by a turbomolecular pump resulting in a substantial increase in density of the trace vapor at the exhaust port of the pump and a corresponding increase in detection sensitivity.